A fuel cell using a solid polymer electrolyte generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen with an oxidant gas containing oxygen such as air. This fuel cell is basically composed of a polymer electrolyte membrane that selectively transports hydrogen ions, and a pair of electrodes, namely, an anode and a cathode, formed on both surfaces of the polymer electrolyte membrane. The above-mentioned electrode usually comprises a catalyst layer which is composed mainly of a carbon powder carrying a platinum group metal catalyst and formed on the surface of the polymer electrolyte membrane; and a diffusion layer which has both gas permeability and electronic conductivity and is formed on the outside surface of this catalyst layer.
Moreover, in order to prevent leakage of the fuel gas and oxidant gas supplied to the electrodes and prevent mixing of the two kinds of gases, gas sealing materials or gaskets are provided on the periphery of the electrodes with the polymer electrolyte membrane therebetween. These sealing materials and gaskets are assembled into a single part together with the electrodes and polymer electrolyte membrane in advance. This part is called “MEA” (electrolyte membrane and electrode assembly). Disposed outside of the MEA are conductive separator plates for mechanically securing the MEA and for electrically connecting adjacent MEAs in series or in parallel in some occasion. A portion of the separator plates, which is in contact with the MEA, is provided with a gas channel for supplying a reacting gas to the electrode surface and for removing a generated gas and excess gas. It is possible to provide the gas channels separately from the separator plates, but grooves are usually formed in a surface of the separator plate to serve as the gas channels.
In order to supply the fuel gas and oxidant gas to these grooves, it is necessary to branch pipes that supply the fuel gas and the oxidant gas, respectively, according to the number of separator plates to be used and to use piping jigs for connecting an end of the branch directly to the groove of the separator plate. This jig is called “manifold”, and a type of manifold that directly connects the supply pipes of the fuel gas and oxidant gas to the grooves as mentioned above is called “external manifold”. There is a type of manifold, called “internal manifold”, with a more simple structure. The internal manifold is configured such that through holes are formed in the separator plates having gas channels and the inlet and outlet of the gas channels are extended to the holes so as to supply the fuel gas and oxidant gas directly from the holes.
Since the fuel cell generates heat during operation, it is necessary to cool the cell with cooling water or the like in order to keep the cell in a good temperature condition. In general, a cooling section for feeding the cooling water is provided for every one to three cells. There are a type in which the cooling section is inserted between the separator plates and a type in which a cooling water channel is provided in the rear surface of the separator plate so as to serve as the cooling section, and the latter type is often used. The structure of a common cell stack is obtained by placing these MEAs, separator plates and cooling sections one upon another to form a stack of 10 to 200 cells, sandwiching this stack by end plates with a current collector plate and an insulating plate between the stack and each end plate and securing them with clamping bolts from both sides.
In such a polymer electrolyte fuel cell, the separator plates need to have a high conductivity, high air-tightness for the fuel gas and oxidant gas, and high corrosion resistance against a reaction of hydrogen/oxygen oxidation-reduction. For such reasons, a conventional separator plate is usually formed of carbon material such as glassy carbon and expanded graphite, and the gas channel is produced by cutting a surface of the separator plate, or by molding with a mold when the expanded graphite is used.
With a conventional method employing the cutting of a carbon plate, it was difficult to reduce the cost of the material of the carbon plate and the cost of cutting the carbon plate. Besides, a method using expanded graphite also suffered from a high cost of material, and it has been considered that the high cost of material prevents a practical application of this method.
In recent years, attempts to use a metal plate, such as stainless steel, in place of the conventionally used carbon material have been made.
However, in the above-mentioned method using a metal plate, since the metal plate is exposed to an acidic atmosphere of the pH of around 2 to 3 at high temperatures, the corrosion and dissolution of the metal plate will occur when used in a long time. The corrosion of the metal plate increases the electric resistance in the corroded portion and decreases the output of the cell. Moreover, when the metal plate is dissolved, the dissolved metal ions diffuse in the polymer electrolyte membrane and trapped at the ion exchange site of the polymer electrolyte membrane, resulting in a lowering of the ionic conductivity of the polymer electrolyte itself. For these causes, when a cell using a metal plate as it is for a separator plate was operated for a long time, a problem arises that the power generating efficiency is gradually lowered.
It is an object of the present invention to improve a separator plate for use in fuel cells and provide a separator plate which is composed of a metal material that can be easily processed, restrained from corrosion and dissolution to maintain chemical inactivity even when its surface to be exposed to a gas is exposed to an acidic atmosphere, and has good conductivity.